POROSITY FACTORS THAT CONTROL THE HYDRAULIC CONDUCTIVITY OF SOIL-SAPROLITE TRANSITIONAL ZONES

Slowly permeable transitional horizons separate soil and saprolite, bu t these horizons cannot be identified easily in the field. The objecti ves of this study were to determine why the soil-saprolite transitiona l zone (BC and CB horizons) is slowly permeable, and to evaluate ways for identifying it in the field, Two saprolite deposits were studied i n the North Carolina Piedmont. At each site, saturated and unsaturated hydraulic conductivities (K-sat and K-unsat) were measured for major horizons. Volume fractions of water-conducting pores were also compare d with the changes in hydraulic conductivity with depth. Horizon mean K-sat values at both sites ranged from virtually 0 to approximately 3 cm h(-1). The lowest K-sat values (<0.3 cm h(-1)) occurred in or near the transitional horizons that were directly below the Bt horizons. Ch anges in the volume of pores within or between mineral grains (termed inter/intraparticle pores) with depth corresponded to changes in both K-sat and K-unsat. In the transitional horizons, the inter/intrapartic le pores were plugged with clay and this caused the horizons to have l ow K values. In situ measurements of K-sat with depth were the most ac curate technique to use for identifying transitional zones in the fiel d. Examination of both the soil and rock structures in pits was also a n acceptable technique. Texture and consistence were not considered re liable for pin-pointing transitional horizons.